1. Field of the Invention
The present invention relates to a telecommunication antenna, used in particular for cellular telephony. This kind of antenna is formed of arrays of closely spaced radiating elements.
2. Description of the Prior Art
Antennas of this type are obtained by means of the printed circuit technique and consist of parallel arrays of dipoles that are placed in a casing serving as a reflector. These antennas, usually called “patch” antennas, are widely used at present because of their very small overall size, their extremely simple fabrication technology, and their moderate cost, because they are mass produced.
These antennas nevertheless are subject to production difficulties because of conflicts that exist between the various design criteria. In particular, although the mutual coupling that can occur between the individual radiating elements when they are close together improves the performance of the antenna, it also has certain negative effects, such as distortion of the spectrum of the antenna or modification of the input impedance of the elements for a given frequency. It is therefore a question of limiting this coupling without significantly increasing either the weight or the overall size of the antenna.
In order to preserve uniformity of radiation, it is necessary to maintain decoupling of good quality between the arrays of dipoles. The arrays of dipoles are usually isolated from each other by simple metal walls forming screens. One solution for obtaining improved decoupling is to increase the height of the screen to block electromagnetic transmission between the elements. However, if the walls are very close together, the radiating elements are confined within a small space created by the screens at which multiple reflections occur that reduce the bandwidth. This degrades the performance of the antenna, in particular the standing wave ratio (SWR), which is reflected in a mismatch between the input impedance of the antenna and that of the transmitter (in the case of transmission). It is linked to the modulus of the coefficient of reflection of the antenna.
To solve this problem, it has been proposed to dispose radiating elements side by side on a reflector, for example. A conductive metal line placed in the same plane as the elements and connected to ground and to the reflector surrounds the radiating elements. The radiating elements and the metal line can in particular be produced by etching a layer of copper covering a dielectric layer.
This embodiment applies only to elements contained entirely within a plane parallel to that of the reflector. This solution is not applicable to radiating elements that are in a plane perpendicular to the reflector, as is the case with dipoles. The mechanical structure to be used in this case is complex and costly.
An object of the present invention is to eliminate the drawbacks of the prior art, and in particular to minimize the reflections that exist between the metal walls of the antennas and the radiating elements, at the same time as maintaining a high level of decoupling without reducing the frequency band.